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Why does the turn tighten in a Spiral Dive as it progresses? Even without applying any elevator control?

Why does the turn keeps getting tighter as the spiral dive develops?
Use of elevator control makes sense as it would increase the horizontal lift vector inside the turn. But even without any elevator control the turn tends to get tighter eventually.

deleting tag "spins"-- feel free to add back if you are convinced this question really does have something to do with spins.

It's a case of the planes "static stability" gone amok as the plane circles downward.
The lift of the banked wing causes a change in direction, which causes a change in the relative wind on the elevator/horizontal stabilizer.  This has the same effect as pulling on the elevator in level flight: increasing the Angle of Attack of the wing ---> which increases lift ---> which further tightens the circle ---> which further changes the relative wind on the tail.
This is why it is imperative, when falling into a spiral, to:

Cut power to avoid overspeed
Reduce elevator back pressure
Use ailerons to roll out of the spiral
Recover from dive
Add power back in

Unintentional spiral dives are rare in VFR, but are all too easy to get into when visibility is limited, and can be started by increasing back pressure on the yoke (in an effort to hold altitude) without realizing the plane is banked.  Learning the attitude indicator and cross checking heading changes can be helpful in avoiding this situation.

Comments addressing this answer have been moved to a chat room-- see https://chat.stackexchange.com/rooms/149712/discussion-between-robert-digiovanni-and-quiet-flyer

It's complicated.  One factor driving the increase in bank angle is that the outboard wingtip is moving around a larger circle radius, and thus experiencing a higher airspeed, and thus creating more lift, than the inboard wingtip.  Which contributes to a rolling-in torque.
However, there are also several factors that are acting to create a rolling-out torque, and for some of these factors, the steeper the bank angle and/or the higher the descent rate, the stronger they become.
Thus the bank angle will generally not tend to increase all the way to (or even approaching) 90 degrees, rather it will eventually reach some limit well short of that.
But, the wings may depart the aircraft before that point is reached.  Especially if a pilot input suddenly increases the angle-of-attack of the wing.  Or if the aircraft exceeds Vne.
And if the aircraft is in cloud and a disoriented pilot is actually contributing a rolling-in torque via an aileron input-- then a very steep bank angle, very high G-load, very high airspeed, and very high descent rate are virtually guaranteed.
PS the main reason that up elevator tends to increase the bank angle is not that a strong horizontal component of lift automatically equates to a roll torque, but rather that a small turn radius, and a lower airspeed than would exist if the pilot relaxed the pitch input, both tend to exacerbate the effect described in the first paragraph above.


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Can an automatic gearbox improve engine transmission for an ultralight?
I am working on my dream to build and fly an ultralight aircraft so I wouldn't like to know the following:

Does adding a motorcycle automatic gear box to an  engine improve on the strength for an ultralight aircraft?  The engine in question is a 4 stroke, OHV, single cylinder, 440cc gasoline engine with a TCI ignition.  It produces 18hp at 3600 RPM,  torque 3.0 kgm/2500 RPM (21.7 lb ft), ignition TCI, gross weight 37 kg.
What should be the tail, flaps and wing measurement proportions for such engine?

Thank you

I disagree with the close votes: while there is an overlap, the linked question does not address part one of the question.

@Hobbes given your answer, I edited the title to highlight that part of the question. Feel free to edit it again if you think I'm missing something.

See also https://aviation.stackexchange.com/questions/8081/why-do-airplanes-not-have-manual-transmission/.

@Hobbes, while I also disagree it is a duplicate of the 18 hp question, we also have a detailed discussion why aircraft don't have shifting gears.

Many engines such as the Rotax 582 used in ultralight and lightsport aircraft do in fact use gears to reduce the prop RPM, and the word "gearbox" is in fact used in conjunction with these gears.  I'm editing the title to help make it clear that this isn't what you mean.

The question is way too broad, hence the close votes, consider deleting the last half and asking it seperately.

A gearbox is not a useful addition. Some aircraft have geared propellers, but that's a single, fixed gear.
The engine has to be able to turn the propeller when it's in the highest gear you'll use, so that determines how much power is required.
In a car or motorcycle, the lower gears are used to match engine speed to wheel speed, allowing the vehicle to drive at lower speeds than 'idle rpm in top gear' without slipping the clutch or the wheels.
In an aircraft, that function is not needed: a propeller can slip freely.

Not teally true, I think.  Motorcycle engines (like all engines) produce more power at high RPMs, with some high-performance bikes redlining above 9,000 RPM.  OTOH, typical GA propellors (and thus their engines) tend to run around 2800 RPM.  So a reduction gearbox would seem to be needed to bring engine RPM in line with propellor RPM, and the simplest way to do this with a motorcycle engine would seem to be to use the transmission, which is typically contained in the same housing as the engine, anyway.

A gearbox would be much heavier than a single gearset. A gearbox also runs the risk of disengaging the selected gear, unlike a gearset that can be splined onto the axles.

That's certainly true, but doesn't take in to consideration that motorcycle engines & transmissions are usually a single unit, so the gearbox is not an addition.  It might be possible to disassemble the engine, remove unnecessary parts, and lock the desired reduction in place.

Some ultralights or lightsport planes with Rotax engines have a feature where the propeller doesn't turn when the engine is at very low RPM. I don't know how the seeming extra complexity of such a feature turns out to be a significant advantage (less work for the starter if it doesn't have to fight the inertia of the prop?) but I've seen it on the Bailey-Moyes Dragonfly.  Could be grounds for another ASE question.

An airplane uses propeller pitch change to achieve the effect of changing gears.
Engine RPM is held constant, the pitch of the propeller is used to take a bigger bite of the air with each revolution, loading down the engine more the same as changing gears does on a car or motorcycle.
So the question becomes:  Is it worth the weight to make the prop pitch changeable (on the ground only, or under pilot control in the air) with a low horsepower engine?

The main point is to maximize the thrust that can be attained with the power available. Having a given power delivered by the engine at at given angular velocity, you should match that angular velocity to an optimum propeller. Thrust is obtained by accelerating a mass of air, and that's precisely the rôle of the propeller. Imagine you have a propeller that accelerates a mass of air X to a speed Y. In terms of energy, the cost of obtaining a given amount of thrust is halved if you double the mass of air and accelerate it to only half the speed. In other words, with a prop accelerating a mass of air 2X to a speed 0.5Y you get the same thrust for one-half of the energy delivered by the engine.
Hence, the bigger the prop, the better. Of course, you are limited by the tip speed, that has to be kept well subsonic, and by the speed to be attained by the aircraft, that should be relatively low. Helicopters use those large rotors for good reasons...
For an ultralight, slow and with little power available, you should use a propeller as big as possible, driven through a reduction gear in order to keep the tips below the speed of sound.

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What is Class E3 airspace?
I am not aware of Class e3 airspace that is now associated in an area I used to be able to fly my ultralight, model airplane, or drone.  It is now apparently requiring FAA authorization.

If this user were still active, I would ask if his problem has been result.  To the best of my knowledge, E3 airspace is currently not in the gridded (authorization-required) portion of the LAANC map for authorization for flying unmanned aircraft (model airplanes, drones).  Also, I would very much like to know if an FAA staffer has directly told him that he needs authorization to fly his paramotor in E3 airspace.  Ping me if you ever come back, FECK, please.

It looks like its an extension of Class C Airspace from a class E3 announcement,

...Class E3 airspace areas are designated as arrival extensions to a
  Class C surface area. Class E arrival extensions are primarily
  designated to provide additional controlled airspace ancillary to a
  surface area to protect instrument operations for the primary airport,
  without imposing additional communications burdens on airspace users.

Do you know how to find out when this became active at a certain airport or airbase?

You may want to get on the FAA eMail list. I get most of my alerts form the AOPA member service alert list which send out local info like this.. My local FBO also generally posts this kinda stuff.

The meaning of E3 airspace is spelled out in several answers to this related question-- What are E2 and E4 airspace?

It would be good to include at least a brief explanation here.

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What transponder code is assigned to Dream Chaser space-plane?
Dream Chaser is scheduled to start flying cargo missions to the space station in 2022. And, Sierra Nevada Corporation claimed it has the capability to land anywhere in the world.I have found it has a landing speed of 200knots. So, it will probably fall in category E as an aircraft approach category.

But, Category E contains only certain Military Aircraft and is usually not included on commercial aeronautical charts.

So, during the landing procedure, What code does ATC assign to Dream Chaser? Is there any special squawk code for the spaceplane? Thank you.

Squawk code and approach category have nothing to do with each other.

"Approach category E" and "Class E airspace" are not related, hence the tag deletion.

The approach category has absolutely nothing to do with the transponder code. If the DreamChaser does have an aircraft transponder, and if ATC assigns it a code, it will be just a regular transponder code that any other aircraft could get.

The space shuttle has a Mode C transponder. There is a high chance Dream Chaser will have but I am not 100% sure.

@Auberron Well, if the space shuttle had one, it's likely DC will have one too. I've taken that part out.


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An airplane has an engine that pushes its flight. What force pushes a glider to fly?
An airplane has an engine that pushes its flight. What force pushes a glider to fly?  Is it gravity? I think the flight of a glider is driven by gravity, but some people disagree with me. They say gravity acts vertical, it has no component in the horizontal direction. Am I wrong?
Please note that this question is not specifically asking about what (thrust-like) force acts opposite to the drag vector.  The direction of a glider's flight path as viewed from the earth may not necessarily be opposite to the direction of the drag vector-- for example, the glider may be travelling horizontally over the ground at constant altitude, or may be rising straight up in ridge lift or wave lift-- and this question may be construed as asking what force is pushing the glider along the direction of the flight path as viewed from the surface of the earth.

Comments are not for extended discussion; this conversation has been moved to chat.

ASE users with voting privileges-- please reconsider your vote. This question should not have been closed. Since a glider can move along a trajectory (as seen from the ground) that is not opposite to the drag vector, this question legitimately invited consideration of forces other than forces that would act along the direction that a thrust vector would act, and thus is not a duplicate of https://aviation.stackexchange.com/questions/56352/what-produces-thrust-along-the-line-of-flight-in-a-glider/  . Some interesting content was posted that would have been off-topic to the other question.

(Ctd)  This includes Peter Kampfs entire answer https://aviation.stackexchange.com/questions/75470/an-airplane-has-an-engine-that-pushes-its-flight-what-force-pushes-a-glider-to/75478#75478 , as well as much of the content in this answer https://aviation.stackexchange.com/questions/75470/an-airplane-has-an-engine-that-pushes-its-flight-what-force-pushes-a-glider-to/75480#75480 . Please respect the hard work of ASE contributors and vote to re-open this question that is not actually a duplicate.

How many times is this question going to be asked?

I still do not see how this would not be a duplicate

@CarloFelicione Because there has been controversy.  Many people think that gravity is not the force of glider movement.

What I'd suggest is going in an searching the archives here for this.  Quite often these kinds of questions have been asked in the past.

The appropriateness of the closure of this question as a duplicate is now a topic on the ASE meta site -- https://aviation.meta.stackexchange.com/q/4067/34686

@CarloFelicione _ I just searched through the archives under "glide" and "glider" and I see no question that duplicates the question as it stood at the time of your above comment, i.e. after the March 22 edits.  I find no question that invites consideration of the problem from the point of view of forces acting along the direction of the flight path as viewed from the ground.  See https://aviation.meta.stackexchange.com/questions/4067/is-the-question-an-airplane-has-an-engine-that-pushes-its-flight-what-force-pu/4071#4071 for more consideration along these lines.

Since this question is obviously not going to be re-opened anyway, I'm tempted to roll back the March 22 addition (which is the entire second paragraph) as it is not obvious whether or not it follows the authors original (or current) intent for the question.  However, noting that he was still engaged on the topic a week later (see his March 29 comment), I guess he would have rolled it back  himself had he so desired, so I'll leave it alone.

@enbinzheng I think you're missing that all aerodynamic forces are defined relative to the arimass. If the air is lifting relative to the ground (e.g. in thermals), the glider will still be descending in the airmass but can be ascending relative to the ground

This question was admittedly rather vague in its original formulation. It would have been better to leave a comment asking the questioner to clarify whether he was speaking about the flight path as viewed from the airmass reference frame or the earth reference frame or both, before leaving any answers. Also to clarify whether or not the question was intended to be confined to steady-situations (constant airspeed and constant trajectory.)

The force that pushes a glider is a component of its weight. More precisely, it's the projection of the vector weight W on the glide path. It's exactly of the same value of drag D, if the glider is flying at a constant airspeed, with no accelerations. In the picture, all vectors are forces except for U, V and w, that are horizontal-, total-, and vertical airspeed, respectively...

$W$ has two components, a component $W_n$ perpendicular to $V$, and a component $W_t$ parallel to $V$. $W_t$ is the force that pushes the glider, corresponding to the thrust that drives a powered plane.

Comments are not for extended discussion; this conversation has been moved to chat.

Airplanes do not fly because of their engines. Airplanes and gliders both fly because their wings turn thrust into lift and drag.
That simplistic answer, of course, just begs the question "how can you generate thrust without an engine?"
A glider is constantly trading altitude (potential energy) for airspeed (kinetic energy). Energy over time equals force, which in this case we call thrust. Airplanes do the same thing when descending. Both can also trade energy in the opposite direction, but only briefly until the wings stall.
The big difference is that an airplane can also turn fuel (chemical energy) into thrust, which allows sustained level or climbing flight. Gliders don't have that option.

Glider turns gravity into thrust, similar to aircraft engines.

The glider turns lift into thrust by having its lift vector point forward. That in turn is done by flying on a sloped glide path. It is actually that lift force which propels the glider.

@PeterKāmpf How can lift tilt forward with a positive AOA?

well, neglegting "small scale" vortices (smaller than the plane), then the airflow vector is the negative of the aicraft velocity vector, they are in the same direction..

@StephenS: Wings do not convert forces, wings deflect airflow. In case of stationary flight (unchanged velocity vector), the forces are in an equilibrium (sum of all force vectors equals zero).  Gliders have no thrust, so that's zero. When gliding, the sum of the weight vector (m*g_vector) and the aerodynamic forces (lift_vector+drag) cancel each other in stationary descent.
And, btw - energy is the integral of a force along a path, no time involved here.

@CarlBerger What if the vector sum of these forces is not zero?  Was the glider pushed in this case?  Who pushed it?

@enbinzheng If the sum of all force vectors isn't vanishing, then there is a net force acting on the airplane mass. That means acceleration in this direction, and the airplane is not any more in a uniform motion. Any flight maneuver would lead to that.

@StephenS Peter Kämpf is wrong.  Lift is perpendicular to the flight path and it is impossible to provide the force to advance the glider forward.

@CarlBerger Peter Kämpf is wrong.  Lift is perpendicular to the flight path and it is impossible to provide the force to advance the glider forward.

Let's do a Gedankenexperiment.
Fly a glider on a horizontal flight path. It will slow down and stall.
Now fly the same glider in a vertical dive. It will accelerate downwards, pulled down by gravity.
Now think of gliding flight as a superposition of both states. Mostly horizontal flight with a bit of vertical dive on top. Shouldn't it be obvious that gravity keeps the glider moving?
But gravity only provides the vertical component of acceleration. The glider in a dive is only accelerated downward, not forward. Only when the pilot pulls on the stick and adds some lift on the wing will the glider experience a forward acceleration.
There you have it. Since the glider is moving along an inclined flight path, its lift vector is tilted forward which provides a bit of thrust. Of course, the lift vector is only needed because the glider flies in Earth's gravitational field, so more gravity or more mass would require an increase in lift, which again means more forward thrust. But gravity is only indirectly involved by setting the lift requirement. What really pulls the glider forward is its forward lift component.
But the glider is not only accelerated forward, but a bit downwards, too. Drag, after all, is also tilted and provides a bit of upward force. That bit needs to be balanced which is done by gravity. In a tilted reference system, it is indeed gravity which is tilted and pulls the glider forward along the tilted longitudinal axis. But if we remain in the standard earth-fixed system, only lift and drag are tilted, and lift pulls the glider forward. Gravity only contributes a downward fraction, the size of which depends on the glide ratio.
Now consider the glider in an upward moving packet of air in which it flies along a flight path which is pointing upwards. But still the pitch attitude of the glider is a bit nose down in order to tilt its lift vector forward. In this condition we now have an upward sloping flight path and still it is only lift which pulls the glider forward.

Suppose the earth has no atmosphere, and you throw the glider horizontally. How does the glider move?  Does it move vertically downwards, or move forward and downward along a parabola?  If you say it moves along a parabola.  So why does it move along a parabola when there is no lift?  Not only that, but it also has acceleration.  why?

Was thinking the same thing, how did they land on the moon?  If you throw a glider or a golf ball (shape does not matter) in a vacuum horizontally, it moves down on a parabola because it accelerates vertically from gravity, while maintaining constant horizontal speed (no drag).  This is why the ungainly LEM "flew" down to the moon from orbit with rocket power and thrusters to safely land.  The modern day Falcon 9 uses drag and rockets to return to earth.  Note, in orbiting, the horizontal velocity is enough for the object to "fall", but never reach the ground.

@enbinzheng - If the glider is in a vacuum, then it is a glider in name but not action. A thrown rock does not glide.

@DeanF. It moves along a parabola because of gravity and initial velocity.

@quietflyer I like your explanation with the glider in rising air. Even in rising air it will keep the nose down in order to tilt its lift vector while its flight path is straight or pointing up. No forward component of the weight vector involved, and still the glider flies forward.

@enbinzheng - “ It moves along a parabola because of gravity and initial velocity.” Let’s define velocity. Velocity can only be defined in vectors (direction and magnitude). If the object being moved/thrown has an initial velocity whose direction is exactly opposite of gravity, it will not move parabolically. It will go straight up, then straight down unless it is acted upon by another force. If the direction of the initial velocity is exactly the same as gravity, it will not move parabolically. It will move in a straight line unless acted upon by another force.

@enbinzheng -   Long distance shooters use this same principle. The vertical acceleration of gravity is a constant. This is regardless of the initial force created by the expanding gasses in the chamber, the mass of the projectile, the muzzle velocity of the projectile, or the shape of the projectile. The arc created by the projectile is not perfectly parabolic due to the fact that although the force of gravity is creating a constant acceleration, neither the force of drag nor the projectile’s velocity is constant. They are calculable. But, they are not constant.

@enbinzheng - “Suppose the earth has no atmosphere, and you throw the glider horizontally. How does the glider move? Does it move vertically downwards, or move forward and downward along a parabola?” So to answer your question..  If you throw a glider in a vacuum, it will move in a parabola. If you drop a glider in a vacuum without any horizontal movement, it will drop like the proverbial stone. The same gravity is acting on the glider in both examples. Its vertical acceleration and vertical speed components will be the same in both examples. The introduction of atmosphere changes the scenario

@DeanF. On the path of the parabola, the speed of the object is also increasing.  So there is a gravity component in this direction.

@enbinzheng - In a vacuum, the vertical component of speed would increase due to the accelerating force of gravity. The horizontal component of speed would not increase at all. In a normal atmosphere this would be different due to the forces inherent with movement through the atmosphere. This is basic physics which is highly documented, demonstrable, and reproducible. This is one of the first labs done in any freshman university physics class.

@DeanF. So when you throw an object horizontally in a vacuum, does it increase speed in the direction of the tangent of the parabola?  Is this the most basic physics problem?

@enbinzheng - When you throw an object horizontally in a vacuum, the object accelerates vertically towards the center of mass of the attracting object. The acceleration is constant. Therefore, the speed increases at a constant rate. In a vacuum, the horizontal speed is constant. It neither accelerates nor decelerates. In a vacuum, the object would travel indefinitely until the vertical component pulled the object to the surface of the attracting object.

@DeanF. Is there any acceleration in the tangential direction of the parabola?

@enbinzheng There's only acceleration directly in the tangential direction if you throw it vertically.  When thrown horizontally, the tangent of the path is going a different direction than the acceleration.  Acceleration points down, but the movement has some horizontal component always because it had initial velocity that doesn't change horizontally.

@JMac The actual path an object takes is a parabola, so the object has acceleration in the tangential direction.  So the acceleration of the object along the tangential direction is increased.

@JMac A parabola is a curved path, and objects moving along the curved path must have acceleration.  Otherwise, the path of the object's movement will not be a parabola.

@enbinzheng The object hasn't accelerated in the tangential direction.  The tangential direction has a horizontal component unless it's thrown vertically.  It's accelerated vertically only.  The horizontal velocity doesn't change.  The acceleration also isn't increased, it's constant in the vertical direction.  If the acceleration was acting along a tangent, it wouldn't even be a parabola.  The parabola is because the acceleration is only vertical while the path has a horizontal component.

@JMac Well, you're right.  So what do you want to explain based on this?

@enbinzheng That there isn't acceleration in the tangential direction.

@JMac I ask you a question: Is there a speed change of the object along the tangent direction?

@enbinzheng There's a change in speed of the object in the vertical direction.  There's no change in the horizontal direction.  I think it would be weird to call that "a change in speed along the tangent direction".

@JMac Don't think it strange, is there speed in the tangent direction?

@enbinzheng I wouldn't say there is a change in speed along the tangent direction, because to me that implies that the acceleration is in the same direction as the tangent; which it isn't.  There is a change in speed in the vertical direction; which has an effect on the overall speed of the object.

@JMac https://physics.stackexchange.com/a/540250/176092

@enbinzheng I've seen that.  I still think it's important to be careful with your wording.  "A change in speed along the tangent direction" heavily implies to me that the acceleration is in the same direction as the tangent.  A clearer way to word it would be "there is a component of acceleration in the tangential direction"; but to me that is quite different than "there is acceleration in the tangential direction".

@JMac There is acceleration in both the tangential and normal directions.

@enbinzheng Again, I think it's much clearer to say there is a component of acceleration in those directions.  The net acceleration is purely vertical.

@JMac Tangential and normal accelerations can replace vertical accelerations.

Let us continue this discussion in chat.

Enbin Zheng is correct, the gravity force is vertical, always vertical, and cannot account for horizontal motion at all. What does?
Let's take the bicycle first.  On flat ground it has a gravitational force of 1 G, but does not move vertically. There for, it has an equal upwards 1 G force of terra firma (the ground) holding it at "ground level". Taxiing aircraft also experience this phenomena, as well as tractor trailers, which is why the lowly semi is around 4x more fuel efficient than the best freighter aircraft (trains outdo us by another 4x). No need for lift here.
Now put the bicycle on an inclined plane, it will "glide" down the hill with no peddling power.  It is the repulsive force of the ground, not gravity, that accounts for the forward motion!  Just draw the vectors.
On to gliders.  No terra firma once airborne (and no peddling).  The repulsive force to gravity is vertical drag.  Every unpowered aircraft has the downward force of gravity pulling it down through the atmosphere.  Every glide starts with falling.  The drag vector, like the gravity repulsive vector of the bicycle, opposes gravity.
But downward velocity is required for vertical drag.  We pay for our drag with the h (height) in the potential energy formula mass x gravity x height.
So now tilt the glider forward by offsetting its center of gravity and center of vertical drag.  (Accomplished in aircraft design with horizontal stabilizer area).  We now have horizontal motion.  A glide slope.  It is only then the more efficient wing can take over lifting duties and "glide" the plane.  Notice the Lift vector from the wing now has a "forward" horizontal component.
Notice also that a plane has much more profile area when viewed from directly underneath than from head on.  This means much more drag is produced from falling a given distance than moving forward.  A little like squeezing  a wet mellon seed between two fingers.  Behold its flight!
A good wing and a well designed glider use this "forward push" with amazing efficiency, as do the greatest of the "motor gliders",  modern airliners.
PS:I have learned to account for forward motion with weight vector when using plane reference. Pitched down, the weight vector can be decomposed to a "thrust" line component.  This matches the logic of converting potential energy to kinetic energy by diving (works for the bicycle too).
Now for Wn.  It has been shown that Wn is the force against the surface of a sliding block on an inclined plane and indeed has significance in determining drag (friction).  As the plane increases, Wn decreases and Wt increases.  As Wn decreases, friction decreases.  Air drag is negligible.  For an airplane at flying speed, air drag is not negligible, and the aircraft is held on its "plane" (glide slope) by Lvertical and Dvertical opposing W. Lhorizontal opposes Dhorizontal in a steady state glide.

Comments are not for extended discussion; this conversation has been moved to chat.

In order for a glider to fly, it must generate lift to oppose its weight. To generate lift, a glider must move through the air. The motion of a glider through the air also generates drag. In a powered aircraft, the thrust from the engine opposes drag, but a glider has no engine to generate thrust. With the drag unopposed, a glider quickly slows down until it can no longer generate enough lift to oppose the weight, and it then falls to earth.
For paper airplanes and balsa gliders, the aircraft is given an initial velocity by throwing the aircraft. Some larger balsa gliders employ a catapult made from rubber bands and a tow line to provide velocity and some initial altitude. Hang-glider pilots often run and jump off the side of a hill or cliff to get going. Some hang-gliders and most sailplanes are towed aloft by a powered aircraft and then cut loose to begin the glide.
The powered aircraft that pulls the glider aloft gives the glider a certain amount of potential energy. The glider can trade the potential energy difference from a higher altitude to a lower altitude to produce kinetic energy, which means velocity. Gliders are always descending relative to the air in which they are flying.
Gliders are designed to be very efficient, to descend very slowly. If the pilot can locate a pocket of air that is rising faster than the glider is descending, the glider can actually gain altitude, increasing its potential energy. Pockets of rising air are called updrafts. Updrafts are found when a wind blowing at a hill or mountain has to rise to climb over it. Updrafts can also be found over dark land masses that absorb heat from the sun. The heat from the ground warms the surrounding air, which causes the air to rise. Rising pockets of hot air are called thermals. Large gliding birds, such as owls and hawks, are often seen circling inside a thermal to gain altitude without flapping their wings. Gliders do exactly the same thing.

NASA: Gliders

As long as there is gravity, there will be gravitational potential energy.  So the thrust of the glider flight comes from gravity.  Right?

@enbinzheng yes. See xxavier's answer, it has a good picture of this.

@Jpe61 His answer seems to be wrong. What is U?  Where does it come from?

@enbinzheng - U is the horizontal component of Lift. See Robert DGV’s bicycle example. Gravity is involved. But gravity can only provide force downward. Forward motion must be provided by a horizontal force. In a non-powered, parabolic flight, the forward motion has already been provided by other means such as the initial launch by hand, rocket, or engines. Without that initial thrust, the flight would be straight up then straight down unless acted upon by another force besides gravity. With an atmosphere, that force is the air itself. Google ballistic experiments and air friction.

@enbinzheng - Another example of this is flight inside uprising air such as an updraft/thermal. A pilot can adjust their  both their horizontal and their vertical speed by changing the vectors of the total lift force. If you change pitch or bank, you will redirect lift. Thereby changing the intensity of the force in the horizontal. Gravity itself has not changed. At a given altitude, it is more or less a constant. The change in velocity in this case is independent of gravity. As a matter of fact, lift can be produced when there is no gravity. All you need is moving air.

@DeanF. The lift is perpendicular to the glider's path, so it is impossible for the lift to pull the glider along the glider's path.  So you are wrong.

@enbinzheng - Lift is perpendicular to the airfoil. The force of lift is divided into vertical and horizontal components in most cases. That is why lift supports less of the aircraft’s weight when the aircraft banks or pitches. Let’s not forget the vectors of lift during a loop or barrel roll. As I mentioned earlier, a good start to your research and argument may be found in the references I listed in an earlier post.

@DeanF. https://www.grc.nasa.gov/www/k-12/airplane/drag1.html The third paragraph reads: "Drag acts in a direction that is opposite to the motion of the aircraft. Lift acts perpendicular to the motion. "https://www.grc.nasa.gov/www/k-12/airplane/lift1.html Note the first paragraph: "Lift acts through the center of pressure of the object and is directed perpendicular to the flow direction. "

"What force pushes a glider to fly?"

It depends on what reference frame we are using.
Newton's first law of motion is Force = Mass times Acceleration.  In linear straight-line flight, acceleration is zero, so net force is zero.  No net force exists in linear (straight-line) gliding flight.
We should then consider the exact meaning of the words "What force pushes a glider to fly?"
These words are basically asking what are all the forces that exist that exert a component parallel to the flight path, acting in the generally forward rather than the generally backward direction.  (Note that the question did not use the word "thrust", which has a different, specific, well-defined meaning in aviation.  "Thrust" is zero in gliding flight.)
The answer to the question depends on precisely what we mean by "flight path".
If we are looking at the flight path through the airmass, there is no such aerodynamic force, but gravity does exert a force component that acts against the direction of the drag vector, i.e. parallel to the flight path and acting in the generally forward direction, even though gravity is purely vertical.  So there's one answer.
If we are looking at the flight path relative to the ground, the answer gets more complicated, and depends on whether the glider is descending, climbing, or maintaining constant altitude.  Consider the case where a glider is maintaining exactly constant altitude in slope lift.  Now what force is providing a component acting parallel to the trajectory, in the generally forward direction?  Not gravity.  And while the net aerodynamic force acts straight up vertically, the component of the net aerodynamic force that we call the lift vector does contain a component that acts parallel to the trajectory in the generally forward direction, and thus opposes the component of the drag vector that acts parallel to the trajectory in the generally backwards direction.  But take care not to confuse this statement with a claim that lift is actually helping to oppose drag-- that's not the case.  Lift and drag are orthogonal (i.e. are perpendicular to each other).
The argument immediately above may strike some readers as an arcane way of playing "games" with components of vectors.  But in truth the same could be said of the claim that gravity is helping to "push" a glider through the air in some way. The key point is that in straight-line gliding flight, lift, drag, and weight form a closed vector triangle, with zero net force.  Which components of this triangle may be considered to be contributing a component that "pushes" forward along the trajectory, depends upon which reference frame we are viewing the trajectory from.
The analysis gets even weirder if we are looking at the trajectory relative to the ground, and the glider is climbing.  There are even valid reference frames where the glider is moving backwards.  Now what is the direction of the force component that we would consider to be purely a "pushing" force?
For example, consider a glider that is slowly ascending straight up relative to the ground in powerful mountain wave lift.  This happens often.  Since the lift and drag vector are always defined relative to the airmass rather than relative to the ground, for a given steady-state airspeed they retain the same orientation in space for a given angle-of-attack of the wing, just as the glider retains the same pitch attitude in space, no matter which way the airmass is moving relative to the ground.  As the glider rises slowly straight up, it's obvious that the lift vector and the drag vector now both contain components that act in the direction of the trajectory relative to the ground, while the weight vector does not.  Similarly, it's not hard to imagine a case where the glider is drifting backwards and climbing along such a trajectory that only the drag vector contains any component that acts along the directory of the trajectory relative to the ground.
The idea that a component of the lift vector is helping to pull the glider forward along the aircraft's trajectory as viewed from the ground is only true when the aircraft's achieved glide ratio is better than the L/D ratio, or when the aircraft is climbing (unless the aircraft is drifting backwards along a climb path that is flatter than the direction of the drag vector).  If there is no tailwind, this means that the air must be rising.
It's usually most useful to focus on the glider's flight path through the airmass rather than the glider's trajectory relative to the ground, or relative to other reference frames, but all of these points of view are technically valid.
When we are using the glider's flight path through the airmass as our reference frame, the answer to the question ""What force pushes a glider to fly?" is "the component of the weight or gravity vector which acts parallel to the flight path."  In this reference frame, neither lift nor drag are exerting any "pushing" force, i.e. any force acting parallel to the flight path and pointing generally forward rather than backwards.
Note that a related question may be asked-- "what powers a glider in flight"?  Work is force times distance, and power is work per time.  Again, the answer will depend on whether we are looking at the work done along the direction of the trajectory relative to the airmass, or the direction of the trajectory relative the ground.  In the former case the answer is simply "the component of the weight vector that acts parallel to the flight path", while in the latter case the answer depends on the direction that the airmass is moving relative to the ground.
Related ASE questions and answers (including some useful diagrams)
What produces Thrust along the line of flight in a glider? (Question)
What produces Thrust along the line of flight in a glider? (Answer)
'Gravitational' power vs. engine power (Question)
'Gravitational' power vs. engine power (Answer)

Your answer is very good.

The lift is perpendicular to the glider's path, so it is impossible for the lift to pull the glider along the glider's path.  So you are wrong.

Momentum and inertia.
The others (gravity, lift, drag) are not sources of energy, but they do modify the forces of energy.  The actual forces involved are momentum and inertia.

Neither of which are forces!

"forces of energy"? My ears are bleeding...

Ooops... SOURCES of energy.  LOL


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Qatar Airways QR161, Jan 10th 2023 - can I expect an official and public report?
Danish media is currently reporting about an incident on January 10th 2023 with Qatar Airways QR161 from Doha to Copenhagen.
Best source I could find (Aviation Herald):
Incident: Qatar B788 at Doha on Jan 10th 2023, steep descent after takeoff
Essential excerpts:

had climbed to about 1800 feet when the aircraft entered a steep descent losing 1000 feet within 24 seconds
the captain took control of the aircraft and recovered about 800 feet above water.
The occurrence was not reported to the authorities and only came to light later.

Official/ public incident report?
By coincidence I started watching Mentour Pilot on YouTube recently, and he often walks through so called "final reports" for various incidents. I wonder if such reports are always made public so that I can expect to be able to read a report on this incidence sooner or later?

1000' of elevation in 24 seconds is 2500 FPM descent. Is that really a "steep descent"? Seems to me that this is within normal descent rates for a commercial airliner. Granted, it's usually not good to do that on take-off, when you should be ascending, but it doesn't seem like it would have caused any significant stress to the aircraft. Maybe that's why it wasn't reported "until later".

@FreeMan 2500 fpm down at that altitude and in that phase of flight is a big deal indeed. The characterization doesn't seem unfair to me. Not to mention that a transport category aircraft ever descending to 800 feet (except during a stabilized approach to a runway) is cause for concern, regardless of descent rate or airframe stress.

Thanks, @TypeIA. I did note that it wasn't good, especially on take off. I guess that's a fair assessment.

And now a very similar incident on United https://www.cnn.com/travel/article/united-777-plunge-takeoff-hawaii/index.html Eerily similar - 2200 ft to 775 ft (vs. 1800 to 800), rest of the flight went normally and flew to planned destination.

That is United Flight 1722. Both that one and this one (Qatar Airways QR161) are now covered by blancolirio (at 07 min 20 secs for the latter (for this question)).

In general, all ICAO member states must investigate accidents and are recommended to investigate serious incidents according to ICAO Annex 13:

5.1 The State of Occurrence shall institute an investigation into the circumstances of the accident and be
responsible for the conduct of the investigation [...]
5.1.1 Recommendation.— The State of Occurrence should institute an investigation into the circumstances of a
serious incident.

(ICAO Annex 13 CHAPTER 5. INVESTIGATION)
The difference between an accident, a serious incident and an incident is also defined in Annex 13:

Accident. An occurrence associated with the operation of an aircraft which takes place between the time any person boards the
aircraft with the intention of flight until such time as all such persons have disembarked, in which:
a) a person is fatally or seriously injured [...]
b) the aircraft sustains damage or structural failure [...]
c) the aircraft is missing or is completely inaccessible.
Incident. An occurrence, other than an accident, associated with the operation of an aircraft which affects or could affect the
safety of operation.
Serious incident. An incident involving circumstances indicating that an accident nearly occurred.

(ICAO Annex 13 CHAPTER 1. DEFINITIONS)
The question is now whether or not the incident you linked to will be classified as serious (see also Attachment C of Annex 13 for examples). According to the Aviation Herald article, the authorities have been informed, in this case the Air Accidents & Incidents Investigation unit of the Civil Aviation Authority of Qatar (QAAI). They will decide whether or not to investigate further. They have in the past closed investigations when the incident was no longer considered serious, e.g.

On Jan 29th 2023 the QAAI informed The Aviation Herald, that following additional information the occurrence was re-categorized an incident, the investigation was cancelled.

(AvHerald - Incident: Qatar A320 near Baghdad and Doha on Dec 23rd 2022, return to Doha results in fuel emergency)
If the QAAI investigates the incident, they will publish the final report once the investigation has been completed:

The State conducting the investigation of an accident or incident shall also make the Final Report publicly available as soon as possible and, if possible, within twelve months.
If the report cannot be made publicly available within twelve months, the State conducting the investigation shall make an interim statement publicly available on each anniversary of the occurrence – detailing the progress of the investigation and any safety issues identified.

(ICAO FAQ: What are a State’s reporting obligations during and after an aircraft accident investigation?)


text:
Plane that always flies into the wind?
Suppose that you dropped an uncontrolled, dummy glider into the air. You want it to always face into the wind by design. Would it be possible? If so, what would such a plane look like?

All airplanes turns into cross wind due to yaw stability (sudden cross wind from one direction is equivalent to a sudden yaw to the opposite, and yaw stability would try to correct that). But the amount of the turn depends on the ratio of wind speed and airspeed, so in order to turn fully into the wind the airspeed needs to be near 0. So you basically need a slow airplane with a huge vertical tail. Or, use a auto pilot and GPS and doppler radar and do it digitally.

How is your aircraft going to know which way the wind is blowing? Will it have accurate navigation avionics? Will it have downward-looking radar? Will it fly only where it can get real-time weather reports?

You're thinking of a kite.

@user3528438,  Airplanes do not turn into a cross-wind. What you are thinking of is that airplanes always turn into the Relative-Wind. That is not the same thing. Cross wind is when the movement of the air mass (the atmosphere) relative to the ground happens to be in a direction not aligned with the aircraft fuselage. Relative wind is just the "wind" you feel from your own aircraft's motion through the atmosphere.

Into the wind relative to the ground, or into the wind felt by the glider?

Face into, or fly into?  If you assume the ability to seat the pilot facing away from the direction of travel (à la "Captain Scarlet"), then your title and body are asking different questions.

@expeditedescent Or a lawn dart, depending on your definition of "wind".

Broadly, how closely it pointed into the wind would be related to the size of the tail plane, in exactly the same way a weather cock works.

Thing about kites, they don’t work well without the string.

The short answer is no.
All the glider feels is its flight relative to the air. What the ground is doing beneath it is not relevant, the wind could be blowing a hundred kilometers an hour and the glider would just be carried along without feeling a thing. This is why unpowered balloons always drift with the wind.
To detect and respond to ground speed requires a sophisticated control system.

@Adullah Your understanding or definition of “wind” in this case is wrong. Wind is just movement of an air mass relative to the ground. In flight there is “relative wind” but that is not the subject of the question.

@MikeSowsun ah, I forgot about Newton's first law. HA HA

suppose you dropped an uncontrolled dummy glider into the air...

Dropped is the key word.  Upon release the glider, if it were directionally stable, should "point" into the "wind".  The key is the definition of relative wind.  This would be air flow relative to the air craft.  Any heavier than air craft must have some relative wind to glide.
A glider creates lift by making its own "relative wind" by converting mass and altitude into airspeed.  However, when it is first "dropped", it only has the force of wind relative to its drop point on it.  A gravity bomb dropped into the slipstream has fins to help it "point" forwards.  An aircraft parked on the tarmac will try to point into the wind, but the friction from its tire prevents it from doing so.  But the common weathervane uses its "tail" to always point into the wind.
So once the plane is moving, the direction it "points" will be in the direction of its "relative wind", which is a combination of its forward motion and any side force winds.
Side force winds will accelerate the plane in that direction, and then and only then, does the plane "not know or care" about wind direction (relative to the ground).  The danger of microburst gusts makes this point abundantly clear.  (There is an opposite effect once the gust passes).

what will this plane look like?

Any aircraft with directional stability

The question isn't very well written and frankly should have been clarified before any answers were posted. But inasmuch as any answer might be posted, clearly a good answer will take into account the momentary effect of inertia, which allows the glider to weathervane into the wind for as long a period it takes before the glider's movement is entirely within the moving air mass's frame of reference. This is the only answer that addresses this!

It seems like the milk bottle would be a huge problem for stability, as it has much more surface area than the tail!

@Peter Duniho, any momentary effect of inertia allowing the glider to weather vane presumes that the drop occurs from a point fixed to the earth.  I agree it isn't a good question since this sort of initial condition wasn't stated.  That is why I included certain presumptions in my answer.

@Harper-ReinstateMonica That is a milk powered engine from the Weight Lifting Cow Company.

"Uncontrolled" is a bit vague.  I am assuming you are referring to wind as measured in the reference frame of the ground.  To face the wind, the glider needs to know where the ground is located at a minimum.  A simple example would be a kite, where the only control is a connection to the ground.  A good kite always faces into the wind.
If you are referring to wind as measured by the glider, all gliders in normal operation fly into the relative wind.

Well, the glider would also have to know which was the ground was moving, not just where the ground is.   Many free-flight and radio-controlled airplanes and gliders have characteristics that keep them basically wings-level even with no help from the pilot, so they arguably "know which way the ground is", but they don't know which way the ground is moving, and show no tendency to face into the wind.  So the answer could be improved by saying something about a need to know which way the ground is moving, or is "trying" to move in in the case of the tethered kite.

This is a good answer, but based on the sophistication of the question I think stating that all gliders fly into the "relative wind" without defining what that means could be confusing.

@quietflyer "where" includes enough information to determine velocity, "which way" does not.

@AnonymousPhysicist  -- re ""where" includes enough information to determine velocity, "which way" does not."-- that depends upon whether you define the ground as a simple featureless flat plane, or something with specific details of which you are apprised of the location of which.  But in general I don't agree with your comment and don't agree that "knowing which way the ground is" implies that you also know which way the ground is moving.  So I think the answer could be improved.

@quietflyer Not sure why I am bothering to respond, but...  "Where" implies a definite measurable location.  The idea of a featureless plane is irrelevant to this, and to reality.  "don't agree that 'knowing which way the ground is' implies that you also know which way the ground is moving. " That is the opposite of what I said.

The answer is no for an "uncontrolled dummy glider".
To explain, I am going to make a few assumptions that aren't stated in your question:  The glider is inherently stable on all axes, it is dropped from a balloon that is drifting with the wind, and the air mass is uniform and stable.
In this case a dummy glider will have no sense of any wind.  Only the person releasing it, and/or a GPS system would realize that there is a track across the ground.  The glider will glide forward through the airmass, (that it is already moving with...) in whatever direction it is pointing when released.
Air is only called "wind" when it is moving relative to the earth.  The reason a kite or simple weathervane points into the wind is because they are fixed to the earth's frame of reference.  A free glider isn't.

Do you have to have information about the ground? No!
Ground tracking is far too expensive. A simple gyroscope that is set just before the drop will do the trick. Search online for miniature gyroscope for model planes (if indeed it is for a model)
Otherwise buy yourself a quadcopter and steal the appropriate bits out of that.

Wind is nothing more than the motion of the ground relative to the airmass.  A plane flies within the airmass and has no way to "know" which direction the ground far below is moving.  So, no, what you propose would generally not be possible.  Exotic schemes based on operations within the wind gradient, albatross-style, could be an exception-- arguably in this case an aircraft does "feel" the wind and there may be some way to harness this effect to drive a direction-pointing tendency.  But certainly not if the aircraft is flying within a uniform airmass that is translating horizontally at a steady rate with respect to the ground.
If the glider is dropped from a high platform that is anchored to the ground, then immediately after it is released its ground speed will be zero and its airspeed will equal the windspeed and it will certainly tend to yaw or "weathervane" to point directly into the wind, but if it has dihedral, it will also tend to roll in the direction that sets the glider up for a banked turn away from the wind direction.  So if we want to design the glider to yaw into the wind immediately after it is dropped, it should have a large vertical fin, minimal rotational inertia in the yaw axis, lots of mass in relation to total surface area, and a mid-wing configuration with no dihedral.  But these characteristics will only produce a temporary tendency for the glider to yaw into the wind, immediately after being dropped.  Eventually the heading will inevitably wander to point in some other direction.

TLDR
We're going to have to break down your requirements and define them to be able to answer:

Plane

so, a heavier than air craft that uses lift to defy gravity

flies

so, "remains aloft for a period of time". What period is acceptable to constitute flight? I'd say indefinite, because you said "always" but..

into the wind

so, capable of moving away from a reference point that the wind blows towards

uncontrolled,

so, either has no controllable surfaces or no device (pilot, autopilot) capable of controlling the surfaces

dummy

so, no innately intelligent/adaptive ability to respond to changing conditions

glider

so, not powered by an engine type device providing constant propulsion

dropped

so, not provided with any initial horizontal inertia to achieve airflow over its lift surfaces

face into the wind by design

so, having an asymmetrical aspect that will rotate it around its balance point while that aspect is experiencing a force from the wind

Would it be possible?

The requirements are likely to be in conflict depending on how you define flight, but if you start high enough and trim your plane well enough, and don't provide starting conditions that are too hard to recover from you might achieve something you define as a plane flying into wind before it hits the ground. The craft could be placed into an airstream above a reference point, and would initially fall as well as being blown away from the reference point. As gravity accelerates it, it could achieve a flight path such that it turns and starts moving towards the reference point again and possibly move past it to end up upwind of it by the time it hits the ground. It would take longer to hit the ground than a free falling object.
Would this be flight?

What would it look like

You've kinda specified already that it has to look like a plane. It wouldn't look exceptionally different to any typical plane you could imagine
The  version:
Are we designing it right now? It might be better to get a handle on the physics involved first..
If we start off with a simple device that turns itself to face the wind, maybe it would look like a windsock. A windsock works really only works because it is anchored to a pole that is anchored to the ground. When the wind blows on it, it experiences wind of some speed relative to the fact that it is fixed in place. The windsock extends in the direction that the wind is blowing and remains thus because the pole is constantly providing a force that keeps the sock stationary, thus it constantly experiences a force from the wind. It is light yet presents a shape that experiences a large amount of friction on the air rushing past it. The air drags it out into a windsock shape
Imagine if the windsock tore loose and blew away. This drag would cause it to quickly accelerate to the same speed as the wind. If the wind were perfectly turbulence free it would retain its shape because it is now an object doing 60 km/h in an airstream also doing 60 km/h but it would fall to ground (and end up looking not like a windsock) because it has no way of defying gravity. If it experienced turbulence it would get kncocked out of shape before it hit the ground. But the idea was there - it's something that can catch the wind and turn to face it.
Let's dispense with the problem of gravity making our stuff crash and talk about something buoyant instead, like a hot air balloon. It resists gravity because of the hot air and starts moving in some direction because the wind is blowing against it, pushing it. Eventually, let us suppose the balloon reaches 60km/h because it's part of an airstream doing 60km/h. It's no longer experiencing any forces that will turn it around or accelerate it because it's doing the same speed as the only thing that as pushing on it; the 60km/h wind
Suppose you really want to turn your craft and you have had some huge paddle on a long pole. Normally it's stashed carefully away in the basket of the ballon, but during flight you assemble it and it ends up looking like an enormous lollipop, and you and stick it out of the balloon sideways, flat side of this massive lollipop turned so it's "catching the wind" rather than "slicing the wind" (lollipop face vertical, not horizontal). Sideways as in, if the balloon is heading East, the lollipop is pointing North or South
If the balloon is already doing 60km/h and the lollipop is doing 60km/h and the wind is doing 60km/h, will the lollipop experience a force that (if it were bolted securely to the balloon basket) will cause the balloon to rotate (so that the lollipop is downwind of the balloon) ?
No ... Because everything is doing 60km/h already. The wind doesn't blow any harder on the lollipop than it does on the balloon. There is no overall force acting on anything in this setup
Suppose you could turn the wind from "blow" to "to suck", like on a vacuum cleaner, at the flick of a switch so that it abruptly reverses direction. Now there is a force acting on the balloon and the lollipop. The balloon is going east at 60km/h, the wind is suddenly blowing towards the west at 60km/h.
The shape of baloon plus lollipop is asymmetric, so the lollipop is catching the wind on one side and there is no lollipop on the other side to balance out the forces.. So the asymmetry of the lollipop will make the balloon rotate so the lollipop is downwind of the balloon.. Why downwind? Because eventually the balloon will rotate to a point where the lollipop is slicing the wind; the wind is blowing towards the East, the lollipop is pointing towards the East. There isn't any part of the lollipop's face that the wind is blowing on, so it experiences no overall force. Because of inertia the balloon will  carry on rotating so the wind is blowing on the lollipop again, this time on the other side. The effect would be to stop it rotating clockwise, and starting it rotating counterclockwise. It will oscillate back and forth in its rotation, and each time it won't rotate quite as far thanks to friction (drag) experienced by the object as a whole.
It might even become stable (stop rotating, because the "into wind" part of the question implies that the direction the craft points be fixed relative to the wind direction) but we need to hope that the balloon finishes its oscillating before this opposing wind manages to slow the whole thing down from 60km/h (going East, say) to 0 and then accelerate it up to 60km/h in the opposite direction (going West, say). If it doesn't manage to stop the balloon's oscillating rotation by the time the wind blows the whole thing so it's doing 60km/h then the entire assembly will stay rotating slowly around forever because we're back to that situation of averything doing 60km/h and there being no overall force
If you think about it, most things that fly with direction are like this balloon and lollipop arrangement  - helicopters, planes, ships, windsocks. That "thing sticking out to provide asymmetry so a force can be appllied to the end of it to turn us around" could be applied to your uncontrolled glider too, but I think you really need to consider what you mean by uncontrolled.
If, like early humans did, we move on from the balloon as a gravity defying device and try and devise a plane shaped thing that meets our requirements, we will probably end up with something shaped a lot like a modern plane. There is a huge variety of shapes but when you look at them from various angles there are some aspects that are symmetrical and some that are not. We generally design them so that when viewed from the top they're symmetrical with respect to the direction they're supposed to fly in, which helps provide some symmetry to the control for rotating them and also helps them achieve a low effort stable and level flight.  Viewed from the side there is an asymmetry that generally helps with both the overall design goals of defying gravity and turning the plane round so it flies in the direction it was designed to. This does all mean that they're engineered to fly well in a particular direction and orientation. Some can fly in other orientations, for example you might be able to balance things out so a plane flies "on its side" and the air smacking into the side of the fuselage keeps it aloft and defying gravity, while the rudder keeps it turned so that the fuselage is always making a suitable angle for the air to smack into and the engines keep pushing it forward fast enough to keep the air smacking the fuselage but it doesn't need considerable amounts of control and power to fly this way
You've set out that it's a glider so doesn't have any power of its own, and it's not even clear whether it gets an initial push in the sight direction because you're just "dropping" it into an airstream, which I take to mean that you're going to throw it out of the back of a larger moving plane, possibly the same speed backwards as the larger plane is moving forwards so that initially your glider has no velocity. It has some unspecified attitude (angle/orientation) that isn't necessarily its stable, level flight configuration (and for the most part probably won't be - I'm assuming you'll want to test how your glider performs in recovering itself when you start it off from an orientation that is unusual. I'll also assume that you're out are so good at throwing it that it has no initial rotations
Gravity affects things significantly right from the start. The wind might or might not affect things much depending on the craft's orientation. There will be some part of the airframe that is catching the wind and will experience a force. If it's in a stable level flight orientation, that force isn't huge because we try and design things so that it isn't, giving us less friction to overcome (costs less to run an engine, flies for longer on its own inertia), but it's  present. Absent any other factors the wind will be blowing over the lift surfaces giving it some ability to resist gravity, but it will dip towards the earth because trimming for stable level flight needs the craft to have some consistency to its speed and it's speed is already changing. Over time things will try to balance out; various factors about the asymmetry of rudder and elevator conspire to turn the craft around so it points towards the direction the wind blows from but the only way it can fly into the wind is to use gravity to overcome the friction the frame experiences from the wind blowing on it, and gravity is in finite supply because altitude is in finite supply; you can't make this craft fly indefinitely so you could say you can only design it to resist crashing as long as possible. It will probably have adopted a flight configuration that means it's pointing into the wind by the time it crashes though
If you make things really hard, give a large amount of rotation in all 3 axes to your magical shaped thing when you throw it out of the hatch of your B52, then there is a good chance that it will never work out; the asymmetry of the rudder and elevator will try their best to reverse some rotation that is going on but there will need to be friction to reduce the oscillations. If you increase the friction you increase the wind's ability to accelerate the airframe. Even if you keep friction low, in this crazily rotating mode the craft will spend a large amount of its time facing in a direction where the wind can push directly on all the surfaces of the craft, particularly the wings and fuselage meaning that the craft quickly accelerates to the speed of the wind and that then means it's just an object free falling under gravity. As it accelerates it picks up speed that may improve the wings' ability to provide lift if the craft is oriented so that they're in a flying direction but overall your glider is really heading in the wrong direction and self recovery to something that could be called flight is unlikely, so even though the rotation is constantly changing you'll probably see it tumbling over and over until it crashes. Deploying a big windsock out of the back end for a while until the oscillations stop, then cutting it loose might work out - but that's getting back to having active control of the craft..
Balloons defy gravity because they're buoyant, planes and gliders defy gravity because they maintain some relative difference in airspeed over their lift-providing surfaces. If you're looking to design something that remains aloft and weather vanes (turns to align itself with the wind direction) I recommend you first find a way for your device to remain aloft and adopt a stable flight (maintains altitude without tumbling around) - this reduces the number of problems you have to solve in terms of 3D movement. Then resolve this problem of it needing to turn into the wind and cease its rotating oscillation sooner than it accelerates to the point that it's doing the same speed as the wind.
It will probably be like a hot air balloon with a light part at the top and a heavy part at the bottom. It will be very heavy so it has inertia but the weight will be very close to its centre of rotation, so a rudder (lollipop) type thing can easily turn it (low rotational inertia) but it is hard to blow it sideways (high lateral inertia). It will have a symmetry to its body and buoyant enough to counteract the core weight.
That does leave us with the problem that you called for not only pointing into wind but flying into wind. No matter what you design, even though it might be able to point into the wind it won't be able to fly into the wind, "flying" defined as "remain aloft indefinitely". An unpowered balloon can't fly into the wind at all. A glider can only fly into the wind while gravity overcomes the friction caused by flight and we know that gliders don't remain aloft indefinitely without some external driving force because the friction they encounter reduces the inertia they need to keep flying or gradually gives them an inertia that leaves them with no airflow to generate the lift needed to keep flying. A propelled plane flies into wind because the engine provides the power to overcome the friction, but only for so long as the engine runs.
This means that the answer to your question as presented, is probably "no", based on what I guess you want

"...gliders defy gravity because they maintain some relative difference in airspeed".  Correct!  This is why you put the drag behind the CG on a directionally stable aircraft.  Lifting is the task of the wing.  Notice a buoyant airship under power follows the same rules (fins in the rear).

In that comment, are you thinking about drag being the device that turns the aircraft into wind? It could.. but it'd be incredibly wasteful and give rise to the problem that it's easier for the wind to accelerate the craft, which we're trying to avoid. The rudder as a device for yawing uses lift (in a horizontal direction) like a wing does rather than drag. A plane deploying some sort of emergency parachute braking device attached to its rear end would be using drag to weathervane into the effective wind it experiences (a combo of the airflow caused by its motion plus the wind direction..

You're on the right track!  Aircraft use less draggy airfoils to "point" into the wind.  We use a symmetrical airfoil as a vertical stabilizer too.  Remember, all heavier than aircraft have to make their own wind (so they can lift) and have to be directionally stable in it.  When all else fails, drag can be used to point (stalled stabilizer with enough area, or a parachute!).

Did you intend for that comment to be so patronizing?

Seldom in professional discourse, which is what this network is about


text:
Is a trimmed flight not always an unstable system?
As soon as I am flying in trimmed flight condition, my pitching moment around the center of gravity becomes zero. So my center of pressure has to be located in the center of gravity, so that there is no resulting turning moment.
But assume, there comes a wind gust, and the angle of attack will increase. This causes the center of pressure to move forward in front of my gravity location. So the aircraft will become instable because the angle of attack will further increase (nose up). The only thing the pilot could do is to use the elevator to trim in pitch.
So in the end, as long as I want to use the principle of "trimmed flight" , I will always have to use control surfaces to counteract the unstable flight condition. So why to care about stability, in trimmed flight it will always come to an unstable flight condition as long as I do not use the elevators?
Can you follow me? Is this true?

Welcome to AviationStackExchange. Why would the CoP move forward of the CoG. No explanation of the forces of flight demonstrate a need for the CoG and the CoP to be colocated for stable, trimmed flight. On the other hand, the Center of Lift may change to provide a torque effect on the aircraft, causing it to pitch.

The pilot can also wait and see what the airplane does. This is a valid alternative to correcting every little deviation. And stability does not mean there is no deviation; stability will only manifest itself after the deviation happens, when the airplane returns to its old state.

You are misusing the word “trim” which is causing confusion.  A pilot does not “use the elevator to trim in pitch”, a pilot uses the elevator to set/reset the desired attitude, then uses trim to reduce the force required to hold it there.  This might seem like a minor distinction to a non-pilot, but it lies at the heart of your question.

Trim is often called “the poor man’s autopilot”. If you can tolerate some deviations in altitude and heading, a pilot can let trim keep the aircraft stable while the pilot performs other tasks.

Be mindful that the CoL doesn’t need to be coincident with the CoG, but can in fact be above it, making the aircraft intrinsically stable even without the stability provided by good aerodynamic design.  In the extreme case of a paraglider the CoG is several metres below the CoL, which is as well, since they would be catastrophically unstable otherwise.

@DeanF. where exactly is the difference of the center of lift and the center of pressure? Maybe you can provide the two definitions (formulae) ?

But assume, there comes a wind gust, and the angle of attack will
increase. This causes the center of pressure to move forward in front
of my gravity location. So the aircraft will become instable because
the angle of attack will further increase (nose up). The only thing
the pilot could do is to use the elevator to trim in pitch.

This is not accurate.  Are you familiar with "free-flight" model airplanes?  Some of them are stable enough to be flown in the middle of the afternoon when the atmosphere is full of thermal updrafts and downdrafts, with no input from a pilot whatsoever, and no automatic control system (autopilot etc).
You are correct that if some disturbance increases the angle-of-attack of the wing, with a conventional airfoil with no "reflex", the center of lift and center of pressure of the wing will move forward, which is destabilizing.
But the tail is also seeing an increase in angle-of-attack, or if the tail is set to fly at a negative angle-of-attack in normal flight, then the angle-of-attack of the tail becomes either less negative or somewhat positive due to the disturbance.  Regardless of which of these is the case in any given instance, the change in angle-of-attack of the tail, which is far from the CG and therefore exerts lots of leverage, causes the center of lift and center of pressure of the whole aircraft to move aft of the CG.  So the aircraft noses down to a lower angle-of-attack, where the center of lift and center of pressure are again co-located with the CG.  That's how pitch stability works.
For more, see this section of John Denker's "See How It Flies" website.
Note-- for simplicity, I'm assuming that the only forces we need to consider are the "upward" or "downward" ones.  I.e., that the "center of lift" and the "center of pressure" are one and the same.  This is not exactly true.  For example, if the thrust line were way above the CG, then it would still be true that the center of pressure (including the effect of the thrust vector) was co-located with the CG when the aircraft was at equilibrium, but the center of lift would have to be a bit ahead of the CG.  If you find this simplification to be objectionable, then just (mentally) strike out "center of lift" from my answer and just leave "center of pressure".
In my answer here, I'm assuming that some sort of disturbance, such as a sudden updraft, has increased the angle-of-attack of the aircraft.  However, there's actually an additional question embedded in your question, which is "will a purely horizontal gust of wind cause a change in the angle-of-attack of an aircraft that is flying horizontally?"  That's a whole other "can of worms" that we may not really need to get into right now, but let's at least "lift the lid" and take a peek.  The answer to that appears to be that the sequence of events is as follows:  gust of wind from front -> increased lift -> upward curvature of flight path (which may temporarily decrease the angle-of-attack slightly) -> nose-up change in pitch attitude to conform to new direction of flight path -> continued upward curvature of flight path and continued nose-up pitch rotation.  All assuming that the aircraft has not yet come to equilibrium with the new velocity of the surrounding airmass at any given point in the chain of events.  It's not obvious that the angle-of-attack is ever increased above the original angle-of-attack.

I think you misunderstand how it works, and how you would respond to changes in wind. Increases in airspeed impact all flight surfaces, including the elevator, so a change in airspeed due to a gust isn't going to create large changes in pitch. There will be some change, however typically the changes in pitch would balance out with the fluctuations. It's rare to get really sudden significant changes in airspeed.
Maybe you are asking is why it's worth trimming an airplane if you'll still be having to make control inputs. Trimming reduces pilot workload and the strength needed for control inputs. If you don't trim you will always be fighting a pitch up or pitch down tendency, which takes energy and concentration. Your priorities are to aviate, navigate and communicate, trimming reduces the amount of effort you put into flying the plane so you can concentrate on the other two. Over a long flight, or a shorter flight in challenging conditions conserving your own energy is important, so it's well worth getting into the habit of trimming.
In calm conditions a trimmed airplane can fly with very little input from the pilot, so you can look at charts or plates, have a drink or just relax while you keep a good visual lookout.
Windy or turbulent conditions are more demanding, and you can't fly 'hands off' as much, trimming is still important because it makes the aircraft easier and less work to control.

The author of the question seems to have a misconception that there is no tendency for an aircraft to return to the trimmed angle-of-attack after a disturbance causes a change in angle-of-attack -- this answer doesn't seem to address that misconception.

That's what I'm trying to say in the first paragraph @quietflyer, I'll edit and have another shot at it when I have time.

Don't overthink it too much.  Simplify in your mind.  Trim is used to pre-set hands-off angle of attack. Static stability forces will focus on regaining the trimmed angle of attack if the plane is displaced from its trim state.
Since pilots use airspeed as a proxy for angle of attack, trim sets hands-off airspeed, as far as the pilot is concerned.  The best way to conceptualize trim in flying is to think of it as a hands-free speed setting dial.
If a plane is disturbed from trim, it will be disturbed from trimed AOA and ultimately speed, and its static stability creates restorative moments that pitch the plane in search of the original trim state.  So if below trim speed/above trim AOA, the plane pitches down, and if above trim speed/below trim AOA, it pitches up.
So the thing to keep in mind is, if the plane is disturbed by a gust, and you make no changes to power or configuration, it will naturally seek to regain the AOA/Speed it was at before it was disturbed.  You can let it go and hands-free, it will seek that trim state on its own, but will take its sweet time, and oscillate about the trim AOA in smaller and smaller excursions until back at trim AOA/speed.
Generally you don't want to wait that long, so it's more efficient to help the natural stability forces along, with elevator inputs, to short-circuit the oscillations. To do this effectively, you need a handy proxy for AOA that can be used, in the moment, as a target to shoot for with your elevator inputs. For this we use pitch attitude relative to the horizon.
So you are flying along nicely trimmed, the plane is flying along hands free minding its own business, pitch attitude set, and a something changes AOA and speed.  Pitch attitude will also have changed. Your objective is to get the airplane back to its trimmed AOA/speed.  Instead of letting it hunt up and down and eventually get back to its trimmed state, you make subtle elevator inputs to regain the original pitch attitude, thereby getting back to trim AOA as soon as possible.  Once you have regained the original airspeed, the plane should be back to and holding the original pitch attitude, and you're back to where you started.
You should be in the habit of using trim to set a hands-free speed for the plane to fly for any stabilized speed you want to fly at (for more than, say, 30 seconds).  If you want to fly at 70kts on approach, don't leave it trimmed at 90 kts and hold elevator pressure; trim away the pressure until it will fly at 70kts hands free.  Same for any other speed condition that is more than a short-term transient condition; shoot for the target speed by pitching to a target pitch attitude, and when on-speed, trim away the stick forces so that speed is held hands-free.  This is how to fly with minimum work load, by letting the plane's inherent stability do most of the work.

I added a graphic to your excellent answer - feel free to remove if you don't feel it's appropriate for the context (or let me know if you want changes). Inkscape SVG is available on my site at https://dl.tyzoid.com/flight-stability.svg

@Tyzoid -- Doesn't the lower graphic actually illustrate a plane that is statically stable, but dynamically unstable?  I think its title is incorrect.

The lower diagram I would say illustrates a dynamically unstable condition, not a statically unstable one. A statically unstable airplane wont even seek to return to it's original state; if neutral, it will just drift around, and if negative, it will seek to "switch ends" (like a weather vane pointing the wrong way).  In the diagram the static response is there - a tendency to return to trim state - but the problem is divergence in the oscillations, a dynamic condition.  Also the use of "level flight" is misleading. It's not a case of level, or up/dn. It's trim AOA. I'm going to take it out.

@JohnK Would it be applicable to add back with changes? I can instead just add the top half of the diagram (which showed the pitch-stable configuration), perhaps with better labels?

No if I want to add a diagram I'll draw one myself.  Thanks.

The main purpose of the tail surface is to prevent the pitch instability of the wing from affecting the whole aeroplane. This is why it is often called the horizontal stabilizer.
In the simplest case, the stabilizer exerts no lift force when the plane is in trim. If the nose lifts, the tail AoA increases and it starts to generate lift. The design is arranged so that the pitching moment of this lift is greater than the pitching moment caused by the wing's CP shift. The tail rises and returns the plane to the trim attitude.
There are many complications which can be introduced, but that is the basic physics of how stability works.

I think most answers has missed phugoid oscillation which is a key word here.
Most general aviation airplanes are designed to be more or less stable in flight once trimmed for the actual situation. This means that if the flight is disturbed somehow, the plane most often returs to a new stable situtation. This goes for all the three inputs: roll, yaw and pitch. In the plane you can try this by giving a short measured input on the controls, letting go and wait out the response.
This stability generally comes at a cost of lower performance. So in high performance airplanes the "natural" stability might be intentionally lower: witness as example the extreme case of jet fighters where modern ones often cannot fly without the computer continously counter effecting the instability.
But back to "naturally" stable planes which most often shows a specific pattern called "phugoid osciallation" in the longitudual or pitch direction. This is one of several oscillation modes a plane can have, but one that is most easy to observe in a GA plane. If the plane gets a disturbance in either speed or pitch the following happens (I assume a traditional tail design + starting pitch down):

The plane pitches down below "steady horizontal".

the speed will now increase
as the speed increases, the down force of the tail will increase and start pitching the plane up (the tail is a wing that pushes down in steady flight, as the plane is pitched down, the attack angle of wind will change and the speed of air over it will increase, giving more "down lift").

The plane will now pitch up, often a bit above "horisontal".

the speed will now decrease
as the speed the decreases, the down force of tail will decrease and start pitching the plane down

Start from 1. again.

This will lead to an oscillation. In GA planes a typical period may be around 30 seconds. (There can actually be several periods concurrently). And this now continues with three possible outcomes:

The phugoid oscillation will decrease and gradually stop
The oscillation may decrease and continue at a constant amplitude. Often this is the desired state when designing airplanes, as totally stable migth cost to much performance.
The oscillation is to extreme to be comfortable or over time increases and leads to extreme pitch changes and you as pilot need to take action.

Over time the trim has to be changed as fuel is consumed and changing balance or the engine changes the output due to air temperature and so on. But for an interval, sometimes 30 minutes in my experience the plane simply flyes itself.

Phugoids are interesting to talk about (and the word is fun to say), but isn't that far beyond the scope of the actual question that was asked?  It appears that the original question is confused about the basic principle of static stability, which is a much more basic issue than dynamic stability/instability.  This looks like a great answer -- but to some other question!

Apart from answering a different question, the explanation is in fact incorrect. There is no pitch balance change in response to speed alone (at least for GA speeds). "A disturbance in either speed or pitch" causes fundamentally different effects between the two. Please see this answer if you are interested in phugoid mode.

Also, phugoid oscillation causes the pressure distribution over the stabilizer to change in sympathy. This causes the elevators, and hence also the control stick, to move back and forth. A hand on the controls prevents it from building up at all, or if a gust does start it up, quickly dampens it down.

I think you have a major misconception about stability. Trim and stability are related but they are not the same thing. A stable aircraft will always seek its equilibrium or trimmed state if disturbed. In a well designed aircraft, the equilibrium is reached by the aircraft automatically without pilot input whatsoever. That is the neatness of it.
In your question, you have not considered from where the stability comes from. It is important to note that in most airplanes, the Center of pressure (CP) should never go ahead of its Center of gravity (CG). This is because most airplanes have a horizontal stabilizer that generates a downward force. If the CP were to go ahead of CG and with a tail producing a net downforce, the aircraft will remain unstable. However, such an aircraft can be stabilized by having a stabilizer that produces a net pitch up moment. With such a system, the larger moment arm of the stabilizer can generate an up force strong enough to stabilize the aircraft.
So, how does an aircraft with a CP behind CG stabilize after a pitch disturbance. It is simple really. The CP as you hava said will move forward (but not ahead of CG). As the horizontal stabilizer is designed for negative lift this increase of wing angle of attack will reduce the negative angle of attack on the stabilizer. Thus, the down force on the stabilizer will reduce and keep in mind the  CP is behind the CG. This will reduce the any upward moments and the aircraft will try to return to its trimmed state.
I would like you to give it a try the next time you go for a flight. For this simple excercise trim the aircraft at a particular speed and angle of attack. Then you pull the nose up and leave the controls and see how the aircraft behaves. A stable aircraft will immediately push the nose down to attain its trimmed state. The amount of time it takes to return to trimmed state can vary ofcourse. This will show how dynamically stable the aircraft is. It is highly affected by the amount of damping in the system. For instance, at higher altitudes the aircraft will have lower dynamic stability due to the reduced aerodynamic damping. Thus, it will take more time or oscillations for the aircraft to attain equilibrium. In a stable aircraft the oscillations should die out in time in what we call subsidence.
The video below demonstrates longitudinal stability.

This answer, like ghellquist's, also confuses long-period (phugoid) stability with the short-period longitudinal (pitch) stability. They are not the same. The latter doesn't invlove speed changes at all. An airplane can be perfectly stable in pitch but dynamically unstable in phugoid motion (esp. gliders). This misconception is common amongst pilots, and the guy in the video reiterates it. To demonstrate pitch stability, he should have given a short pitch impulse and return the yoke before speed dropped. For pure phugoid, it's better to change thrust for a few seconds and restore it.

Also, the requirement to have CP behind CG for stability is also a common misconception. CP can be ahead, and the tail can be lifting under certain conditions (and the airplane be statically stable). Generally, CP is not a helpful concept to explain stability; Aerodynamic Centre (aka Neutral Point) is.

@Zeus that is exactly what I said. As long as the tail is designed to generate an up force, you do not need the CP to be behind the CG.


text:
Lindbergh's panel-- Ryan NYP-- what is this instrument and how was it useful?
Lindbergh's panel-- Ryan NYP-- what is this instrument and how was it used?
See this link-- it should allow you to zoom in by mousing around
https://www.amazon.com/Instrument-Successfully-Atlantic-Charles-Lindbergh/dp/B077RBRZTT
Consider the "T" shaped instrument at the lower center--
The top (horizontal) part is a bubble level -- it acts exactly like a slip-skid ball except the bubble deflects in the opposite direction as a ball would--
The lower (vertical) part is some sort of pitch level.  It is calibrated to show pitch attitude.  The glass tube may possibly have a part that we can't see, that disappears into the panel.
So why how was the vertical glass tube intended to be used?  Was it intended to provide useful information in cloud?  If so, would it actually have provided any useful information, or not?  Or was it just intended to display a precise measure of the aircraft's pitch attitude and thus angle-of-attack, during unaccelerated straight-and-level cruising flight in visual conditions, for the purpose of optimizing the airspeed and angle-of-attack for long-range cruising flight?
The calibration suggests that the instrument was intended more to detect large deviations than for fine-tuning the angle-of-attack in cruising flight in visual conditions, but it is hard to see how this would be of much practical use during actual cloud flying.  It's pretty much analogous to a simple pendulum, but with more damping.

Sorry, bum link needs replacement.  Need a good photo clearly showing the instrument (see photo in answer) actually mounted in place in panel.  Ideally we need to find a photo that may be included in the question.

Its an Inclinometer which provided pitch and roll information

In the early 1920's, the US Government contracted Rieker Instruments
  Company to produce bubble style pitch and roll indicators for the fast
  emerging aircraft market.
One of the most famous inclinometer fluid filled vial type
  installations - if not the most famous - was the use of a pair of
  Rieker glass tube instruments on the panel of the Ryan NYP "The Spirit
  of St. Louis".  In 1927 Charles Lindbergh chose the lightweight Rieker
  P-1057 Degree Inclinometer to give him climb and descent angle
  information.

(source)
This was useful to the Spirit of Saint Louis as it had effectively no forward visibility with the exception of the periscope. He may have also chosen this in the interest of weight as it would be much lighter than an early gyroscopic instrument.

Right.  Its just that, based on my experiences, I can see how the slip/skid information would be somewhat useful-- the ball tends to ride to the low side of a bank/ turn, and the bubble to the high side -- but seems to me the physics are such that the pitch indication would be completely useless at any time that you can't actually see out the window.  Now, granted, the Sp of St Louis had a lousy forward view, so maybe that had something to do with it-- even in visual conditions where there are no crazy accelerations going on, a guide to climb/ dive angle might be useful?

PS I have used something exactly like that upper part on a lightsport plane AND a hang glider, many times.  Just to satisfy my curiousity-- doesn't serve much purpose in a rudderless aircraft--

PPS it goes without saying that NEITHER pitch nor roll information is actually provided, unless the aircraft is known to be in an unaccelerated 1-G condition.

Good historical answer though.  Thank you.

Plus it didn't weigh much so there's not much to lose.  Panel space was not really at a premium in this case--

I read Lindberg's book on the trip but I don't recall if he mentioned what the inclinometers were for.  I would assume the longitudinal one was for knowing pitch attitude in unaccelerated flight for the purpose of correcting fuel quantity indications with the very large tank in front of the cockpit.  Lindberg's main problem was constantly drifting off to sleep and wandering off course toward the end of the first night, and hallucinations from fatigue.  When he got to Paris he had enough fuel and considered continuing to Rome, but the thought of a night flight over the Alps changed his mind.